I don't see how it's on Apples side unless the downloaded file is corrupted. In that case, delete it and DL again? Or unless it's at the point of activation.
Mentioned above that it could be firewall? Hmmm, I don't know about that. If that's possible, sometime disabling a freewill doesn't unblock all ports, they may need to be added to the exception list and leave firewall on.
The iTunes installer actually installs a bunch of different programs. I've seen it skip some of these on windows, don't know why, could be a permissions thing. The fix for that is to extract the individual programs out of the package and install one at a time.
Just throwing out some ideas.